Compact radio frequency ("RF") transmitters are widely employed in connection with remote signal communication systems. Compact transmitters are commonly used for remotely controlling automatic garage door systems, electronic sound systems, televisions and VCRs. In the automotive industry, compact transmitters are commonly used in remote keyless entry systems to provide remote control access to a vehicle, as well as controlling other vehicular functions such as alarm system features, trunk release, for example. Ideally, compact hand held transmitters are battery operated, energy efficient and intended to accommodate a compact enclosure.
In one known compact remote system design, the transmitter radiates an RF signal with a predetermined carrier frequency encoded according to an on/off switched pattern. This radiating signal is subsequently received by a remote receiver. Once received, the signal is processed, if necessary, and then provided as a control signal to control a function or feature of the system.
Currently, a number of compact remote RF transmitters employing a single oscillator system for providing a local oscillation signal are known. In light of their cost and simplicity, single oscillator circuits have been the transmitter component of choice in automotive, remote controlled, keyless entry systems.
Single oscillator systems are well suited for the RF signal transmission applications of a remote keyless entry system. However, these known single oscillator designs have several limitations regarding power output in particular applications. Traditional single oscillators, moreover, are overly sensitive to unwanted parasitic impedances created by the grasp of a user's hand on the transmitter's housing, the housing itself or the material to which the transmitter is fixedly adjoined, such as an automobile headliner or visor. This sensitivity is attributable to the additional impedance created by these parasitic effects which reduce the amount of transmitted energy towards the receiver due to the limited power available from these known single oscillator designs. In certain environments, such as those dictated by the European market, an output signal with a higher signal to noise ratio, and relatively greater power strength is required. In other applications, the remote keyless entry ("RKE") systems requirements driven by the international customer necessitate a low power design. In powering down the known balanced oscillator designs, the gain margin is reduced to the point where the small reductions in the gain of the transistors or output tank center frequency prevent the overall operation of the oscillation of the circuit. Thus, too much power would be radiated for particular markets, such as Japan, by a providing a minimally operational balanced oscillator, and in other markets, too little power would be generated by traditional means.
In view of these problems, a demand exists for an oscillator circuit for use in a transmitter having a diminished power output. A need further exists for an oscillator circuit having an increased signal to noise ratio.